1. Field of the Invention
The present invention relates to a photometric device to measure the luminance of an object. The photometric device can be used in an auto exposure control of a camera.
2. Description of Related Art
FIG. 10 is a graph representing the output characteristic V versus the brightness L for a conventional photometric device. Under ideal conditions, the L v. V graph should show a proportional relationship, as illustrated in curve 8-1. However, due to changes in temperature and power source voltage and the like, the L v. V characteristics deviate from a proportional relationship, as illustrated in curve 8-2. The ideal L v. V characteristic is described by Equation (1), for curve 8-1 and by Equation (2) for curve 8-2. EQU v=a.multidot.L (1) EQU V'=a'.multidot.L+b (2)
where a is the slope of the line 8-1, a' is the slope of the line 8-2, and b is the offset of the output characteristic V between line 8-1 and line 8-2.
Therefore, in curve 8-2, the deviation with slope a/a' and offset b, needs to be corrected, based on the obtained value V' from Equation (2). Such a correction can be accomplished using Equation (3). EQU V=(V'-b).multidot.(a/a') (3)
A conventional photometric device is disclosed in Japanese Unexamined Patent Publication Hei 6-208154. This photometric device, which is illustrated in FIG. 11, conducts photometric at intervals of 10/N ms (milli-second), where N is an odd integer other than 1. The photometric device obtains a final photometric value by averaging the latest N photometric values, so the photometry is not effected by light source flicker or variation.
However, with this and other known photometric methods and devices, the required multiplication and division, required in Equation (3), causes a heavy burden on the control assembly. Computation takes too much time, and thus the photometric is not efficient.
Moreover, with the conventional photometric device of Japanese Unexamined Patent Publication Hei 6-208154, the final photometric value will not be obtained, unless photometry is conducted at least N times. Further, N photometric values have to be stored. Therefore, the conventional photometric device and the associated method place a large burden on the memory. Therefore, the photometry is no longer efficient.